The present invention is directed to a composite yarn formed of an elastomeric yarn and a multifilament partially drawn non-elastic yarn and, more particularly, to a composite yarn formed by a process in which the partially drawn non-elastomeric yarn is friction twisted together with the elastomeric yarn during a false twisting operation under such conditions as to maintain a "stable" or "balanced" thread line. Fabrics and garments formed from such yarns have improved elasticity, stretch or comfort properties.
The term "elastomeric yarn", as used herein, is generally understood to mean a yarn having a high degree of stretch, for example, greater than 100% of the original length, which yarns are typically formed of polyurethane filaments, and generally referred to as "spandex". In the same manner, the term "non-elastomeric yarn", as used herein, generally refers to a yarn having a relatively low degree of stretch, e.g., less than 50% of the original length, and is typically formed of filaments of a firmer plastic polymeric material such as nylon or polyester as is generally used in the manufacture of plain or textured yarns. The terms "partially oriented", "partially drawn", and "incompletely drawn" yarns are all synonymous and are defined as filament yarns in which the draw ratio is less than normal resulting in only partial longitudinal orientation of the polymer molecules. Such yarns are only partially drawn by the fiber produced and hence must be finished before or during the texturizing process by the throwster. Composite yarns possessing elastic properties and consisting of elastomeric yarn associated with various types of non-elastomeric multifilament yarn are known in the art. Further, numerous prior art methods and apparatus have been proposed for forming composite yarns of the aforesaid general type.
At the outset, such composite yarns were formed by a wrapping process in which the non-elastomeric yarn wound on a bobbin mounted on a rotating hollow spindle is wound around the rubber or polyurethane elastic core filament as the elastic yarn is drawn through the hollow spindle. This process provides an excellent result, however, is extremely expensive and relatively slow. In accordance with a second process (referred to as "air entanglement"), the elastic core yarn is stretched and a plurality of substantially inelastic filaments are intermingled therewith by a pressurized fluid entangling means (air) to cover the yarn. While this process also results in an acceptable product, it is a process that is generally carried out separately from the texturizing process and, therefore, increases the cost of the resultant composite yarn.
U.S. Pat. Nos. 3,807,162 and 3,921,382 are both directed to a process for making a covered elastic yarn during the texturing process. An elastic core yarn and thermoplastic multifilament sheath yarns are supplied to a false twisting machine in parallel arrangement and subjected to a false twist crimping operation to entangle or intermingle the yarns. In these patents, the direction of twist of the sheath yarn about the core yarn is reversed irregularly and intermittently to provide a resultant yarn that is substantially twistless.
In a similar approach described in U.S. Pat. No. 3,991,548 to Toronyi et al. and a published European Patent Application No. 90305285.0, a composite yarn formed of a thermoplastic wrapping yarn is twisted around a spandex core yarn during a false twist operation in a series of alternating "S" and "Z" twists. In the approach of the European application, the thermoplastic wrapping yarn supplied to the process is a partially oriented yarn, which is completely drawn during the texturizing and twisting process. During the development of the processes described the European application, cohesion between the two yarns forming the composite yarn is a concern. Without adequate cohesion, fabrication of the resulting composite yarn in to woven or knit articles becomes extremely difficult, if not impossible. In order to provide sufficient cohesion, this approach, as part of the invention disclosed therein intentionally introduces parameters that result in "real twist" or permanent twist caused by an unstable or unbalanced thread line which includes a substantial number of lateral loops and voids. Such parameters as twist, D/Y ratio, heat, yarn draw tensions and machine speed were adjusted to achieve the resulting real twist (i.e., voids/tight spots). In the Toronyi et al. setup, the non-elastomeric yarn is undrawn nylon. As such, the setup only works with undrawn yarns because Toronyi et al. uses common feed rolls for both yarns and because undrawn nylon or undrawn yarns and spandex have approximately the same draw ratio.
While the aforedescribed processes have achieved some degree of acceptability in the market, there are some problems which have been noted. Primarily, the provision of real twist (intentional alternating "S" and "Z" twists) as a result of twist slippage results in loops which project laterally from the yarn and voids. Lateral loops and voids form sections of yarn which are objectionable during later processing into fabric and garments. For example, in sheerer fabrics the loops/voids cloud the appearance and increase the likelihood of picks. While Toronyi et al. apparently provides a composite yarn without real twist, it does not provide a technique for accomplishing such a result where the non-elastomeric yarn is partially drawn.
In the present invention, on the other hand, a different approach is taken. Applicants have discovered that by increasing the inserted twist (false twist) to a t.p.i. in the range of 160-200, and adjusting the yarn processing conditions, a more uniform yarn can be formed with a stable thread line (i.e., without alternating "S" and "Z" twists which result in lateral loops and voids). Thus, such objectionable "real twist" is essentially eliminated, or at least minimized. Such variable processing conditions which may be adjusted include disc stacking, draw ratios, D/Y ratio, heat, and speed. While the elastomeric core yarn and partially oriented non-elastomeric yarn of the present invention are both supplied to the heater and false turns section under tension, the twist per inch is greater (160-200 t.p.i.). In order to accomplish this result, adjustments in yarn draw tensions, D/Y ratio, heat, and disc configuration are also adjusted. The resulting yarn, because of the absence of lateral loops, may be knit at increased speeds and reduced tension which improves knitting efficiencies and results in a more uniform fabric. It is also believed that the composite yarn speed through the texturizing machine may be increased to the range of 570-800 meters per minute, resulting in further economies. Further, the higher inserted false twist results in proper cohesion substantially without voids and lateral loops and improves the fabric appearance.
It is therefore an object of the present invention to provide an improved method for manufacturing a composite yarn of the type which comprises an elastomeric yarn cohesively intermingled with an non-elastomeric multifilament in such a manner as to exhibit a stable thread line and minimal real twist.
It is a further object to provide a composite yarn which comprises an elastomeric yarn cohesively intermingled with a non-elastomeric, multifilament yarn, the composite yarn having a stable thread line and having improved processing characteristics when knit or woven into a fabric.